Bathroom Exhaust Fan Size Calculator (CFM)

The Home Ventilating Institute (HVI) recommends 1 CFM per square foot for bathrooms up to 100 sq ft (about 8 air changes per hour). An 8 ft x 5 ft bathroom is 40 sq ft, so the HVI guideline is 40 CFM. However, mechanical codes (IRC / ASHRAE 62.2) require a minimum of 50 CFM intermittent exhaust for bathrooms, so this calculator recommends a 50 CFM fan — the larger of the two values.

For bathrooms above 100 sq ft, HVI switches to the fixture method: count 50 CFM for each toilet, shower, and standard bathtub, plus 100 CFM for a jetted tub. HVI's own example — a 20 ft x 12 ft bathroom with a tub, a shower, and an enclosed toilet — needs 50 + 50 + 50 = 150 CFM. You can install one 150 CFM fan or split it into smaller fans over each fixture.

Yes. HVI's 1 CFM per sq ft rule assumes an 8 ft ceiling. For ceilings taller than 8 ft, this calculator applies a volume correction based on 8 air changes per hour: CFM = volume / 60 x 8. A 10 ft x 10 ft bathroom with a 9 ft ceiling has 900 cubic feet of volume, so it needs 900 / 60 x 8 = 120 CFM instead of the 100 CFM the area rule alone would give.

Every bend and foot of duct adds static pressure, and a fan rated 110 CFM at 0 static pressure can deliver far less once installed. As a rule of thumb (not an engineering standard), count each 90-degree elbow as about 5 ft of equivalent duct and keep total equivalent length under about 25 ft. Example: 25 ft of straight duct plus two 90-degree elbows = 25 + 2 x 5 = 35 ft equivalent — over the limit, so this calculator suggests upsizing the fan about 20% (e.g. 80 CFM becomes a 100 CFM unit) or using larger, smoother duct.

Sones measure perceived loudness: 0.5 sones or less is a whisper, up to 1.0 sones is quiet, up to 1.5 is moderate, and above 2.0 is noticeably loud. For a bathroom you will actually leave running, choose 1.0 sones or lower. ENERGY STAR bath fans must be rated at 2.0 sones or less, but premium quiet models reach 0.3 to 0.5 sones.

HVI recommends leaving the fan on for 20 minutes after using the bathroom so moisture is fully cleared and mold cannot take hold. A countdown timer switch or a humidity-sensing fan handles this automatically — simply flipping the switch off when you leave the room usually stops ventilation too soon.

ASHRAE 62.2 calls for continuous whole-house mechanical ventilation of 7.5 CFM per person plus 1 CFM per 100 sq ft of floor area. For a 2,000 sq ft home with 4 occupants that is 7.5 x 4 + 0.01 x 2,000 = 30 + 20 = 50 CFM running continuously. This is separate from the intermittent spot ventilation your bathroom fan provides — use the bonus mode in this calculator to check it.

It varies by state and locality — always check your local code. The International Residential Code generally requires bathrooms to have either an operable window or mechanical exhaust of at least 50 CFM intermittent (or 20 CFM continuous). Some jurisdictions, such as California's Title 24 and several northeastern states, require mechanical ventilation even when a window is present. Treat this calculator's result as a minimum sizing guide, not a code compliance certificate.